Performers dressed up as characters from the Moomin book series pose at Moominworld, a theme park in Naantali, Finland. Besides saunas and Nokia phones, the Moomin universe may be Finland's most successful export item ever. (Jenni Virta/AFP/Getty Images)

It’s raining gently, and I’m standing in an outdoor theater, in the middle of a crowd of children and parents. Onstage, an actor in a giant, square-shaped fur suit with a red triangle nose lumbers from foot to foot and shakes his head, waggling the pink tentacles atop it. Another man, in a police officer’s uniform, is making officious gestures, while a woman with long red hair, wearing a peaked red cap and red robes and with whiskers painted on her face, speaks squeakily into a microphone in Finnish, a language I do not remotely understand.

This is not some sauna-induced nightmare: It’s the afternoon’s theatrical offering at Moominworld, an amusement park in the coastal Finnish town of Naantali. The actors are playing minor characters in the Moomin stories, a series of children’s books by Tove Jansson, a Swedish-Finnish author and artist who would have turned 100 this month. (She died in 2001.) The Moomins themselves — a family of three: Moomintroll, Moominpappa and Moominmamma — are hippolike creatures with ballooning snouts and tubby bellies, always setting off on bold quests or making discoveries.

In June, I set off on a quest of my own, traveling through Finland in search of the Moomin-land I’ve loved since I read the books in English translation as a child.

My tour begins in Helsinki, the sea-air-scented city — full of turreted art nouveau apartment buildings and trams speeding around every corner — that was Jansson’s lifelong home. On my first day in town, I leave my hostel on Uudenmaankatu and walk north to the grassy Esplanadi park to find two sculptures by Jansson’s father, Viktor. Modeled after Tove, they’re art nouveau mermaids cavorting with fishes, one at the eastern end of the square and the other decorating the outdoor seating area of a nearby Indian restaurant. I don’t love the sculptures — they’re blocky and a little awkward, with conical breasts like the Tin Man’s hat — but I like the resolute fierceness on their faces.

Later that day, I see the same fierce grin in a series of photos of Jansson at the Ateneum, the Finnish art museum that has staged a major Jansson exhibition in celebration of the centenary of her birth. The show encompasses everything from her courageous wartime anti-fascist cartoons to intimate paintings of her friends and lovers to an ecstatic video showing a 50-something Jansson dancing alone in a fisherman’s sweater by the sea. The exhibit also traces the rise of Moomintroll, who began as a cartoon on an outhouse wall and then became a regular figure in Jansson’s cartoons before finally blossoming into a character with an entire world of his own.

Not to mention a brand. After the success of an animated cartoon show in the 1990s, demand for Moomin-related paraphernalia exploded. As I explore Helsinki over the next couple of days, Moomins pop up everywhere like blobby white mushrooms: a Moomin calendar and Moomin stickers decorating the wall at my hostel, Moomin lollipops for sale at the supermarket register, Moomin mugs sold by the fancy Finnish ceramic-ware brand Arabia. At the Moomin Store in Helsinki’s Forum Mall, you can buy Moomin-branded digital thermometers, chewing gum, ties, napkins, bandanas, lip balm, playing cards, cake forms, tea towels, baking trays — and much more.

Jansson herself, though no fragile Finnish flower when it came to marketing, eventually became sick of the characters. “I could vomit over Moomintroll,” she wrote at one point. After the last chapter book came out, in 1970, she turned mostly to novels and short stories for adults. So the next stop on my trip was the place where she went when she wanted to hide from Moomin-mania — the Pellinki archipelago, a group of islands in the Gulf of Finland. It was “the landscape I love more than all others,” Jansson wrote, “and no other place has contributed more to my work.”

Island-bound

The summerhouse that Jansson shared with her longtime partner, artist Tuulikki Pietilä, is on Klovharu, an island far out in the archipelago. I decided to simulate the Klovharu experience by renting my own private island for the weekend through the visitors’ Web site of Porvoo, a mainland town. From Helsinki, I take the bus to Pellinki village, a two-hour ride with a bus driver who stops regularly to smoke tiny cigarillos by the side of the road. Lasse and Anne-Maj, the owners of the island, meet me in the dirt driveway in the center of the village. It turns out that Anne-Maj, an elfin woman in clogs who runs a flower store, knew Jansson well. In fact, Anne-Maj had her first taste of alcohol at the age of 12 at a party on Klovharu. “Why can children not have fun, too?” Tove said, according to Anne-Maj, and poured her a drink.

Once I’ve stocked up on food at the village shop, Lasse, a weather-beaten man with a constant half-smile and a knife on his belt, loads me into his motorboat and steers to what I’m already thinking of as “my” island, a tiny landmass covered in Scotch pine and soft moss, edged with lichen-encrusted rock, and inhabited only by sea gulls and terns. The cottage, like most Finnish summerhouses, is painted barn red with white trim and has sloping wide-board floors and low ceilings that I bump my head on. I watch Lasse’s boat pull away and think of Moominpappa surveying his island in “Moominpappa at Sea”: “This island of mine is a complete world of its own; it has everything and is just the right size. How happy I feel! I’ve got the world in my paw!”

As the sun sets on the first evening, while I’m eating dinner at a wooden table in a small field of purple-and-white wildflowers overlooking the water, the island does seem quite complete. In the morning, though, things have shifted. I didn’t buy tea in Pellinki, but now I have an addict’s desperation: I must have tea! I rifle unsuccessfully through the kitchen and finally boil one of the caramel candies I’d picked up at the store. The resulting brew is cloying but somehow tea-ish enough to satisfy my thirst.

The island may not have tea, but it does have a sauna. So after I’ve drunk my candy-tea, I carry a basket of logs down the pine-needle-strewn path to the sauna and test out my stove-lighting skills. The fire catches, the square iron stove heats up, and soon I’m stretched out on the wooden benches, dripping with sweat, ladling water onto the stones every now and then to make a fizz of steam.

After the sauna, the island feels complete again. I spend a lazy afternoon in the sun reading, pitching darts at a dartboard that I find in a shed, and exploring the island’s rocky coves. Sitting in the kitchen at one point, I hear a great splashing and flapping, as if someone is whacking the ocean with a bat, and I look out to see giant white wings: a swan taking off in flight.

As I’m drinking my pseudo-tea the next morning, I hear a boat motor up to the island. It’s Lasse. Would I like to see Klovharu? he asks.

It’s a long journey out to the open ocean, and the islands grow fewer and starker. Klovharu, the remote island that Jansson called her “angry little skerry,” looks like two smooth humps of rock. Lasse tells me that we’re about 25 miles from Estonia. I can’t see anything on the flat gray-blue horizon except sea birds and a distant bank of white clouds. We tie the boat up in a small inlet and jog up a wooden walkway to Jansson’s gray cabin.

Here I see what completion truly means. The house is a compact one-room cube, designed with windows facing each direction. The bed is a platform covered with a rough woolen blanket, beneath rows of bookshelves. The kitchen is a corner behind the bulbous green stove, with rows of shelves for spices and pots and a hook for a Marimekko apron. There’s no electricity, just candles and oil lamps, and the water is what’s collected in the blue plastic rain barrel out back. Every hook has a distinct purpose; every inch of shelf is used. It’s like a ship’s cabin designed by Ikea.

On the way back, Lasse pulls the boat up at a dock where we buy a half-pound of cold smoked salmon, which I eat on my last night on the island, sliced onto brown bread with cucumbers.

Mad about Moomins

It’s jarring to go from the wild peace of Pellinki to Moominworld, the epicenter of Moomin madness. But Moominworld, in the Old World town of Naantali, about two hours west of Helsinki, is a surprise, a thoroughly Nordic version of a children’s amusement park. There are no rides. The main diversions are long, winding walks through the woods. At intervals, you encounter stations that represent moments from the Moomin books — the witch’s house, or Moominpappa’s ship — and you can slide down a twisty slide or play with buckets and scoops in a pool of water.

My favorite stop is a storytelling area sheltered by pines and filled with hammocks and beanbag chairs, one of which is holding a massive blond man in white shorts who’s fallen asleep. There’s a sandy beach, a few intrepid kids splashing in the frigid water, and two stages, where the incomprehensible (to me) plays are performed to general applause.

At the center of Moominworld is the Moomin house, a huge blue tower with a pointy red top. As I approach, there’s a sort of lurch in the crowd, and I see why — the Moomins have arrived! Children are swarming them: grabbing their tails, snuggling into the plushy stuffed bellies, stretching their arms wide as if trying to hug a redwood.

In the Venn diagram of Moominworld visitors, there are Finnish families and there are the Japanese (who are so obsessed that they recently created a Moomin Cafe in Tokyo where you can eat your meal opposite stuffed Moomin characters). Then, outside both those circles, there’s me. I feel totally abashed as I sidle up to the giant white-snouted Moomintroll, smartphone in hand, and gaze into its clear plastic eyeballs.

But there’s no way I’m leaving Finland without a Moomin-selfie.

Peterson is a Washington writer and the Word columnist for the Boston Globe Ideas section.